Fiber reinforcement

ABSTRACT

A FIBER REINFORCEMENT HAVING KNOB-LIKE END PORTIONS AND COMPRISED OF AN INORGANIC FIBER COVERED WITH A CHEMICAL VAPOR-PLATED COATING OF GREATER THICKNESS AT THE ENDS THAN THEREBETWEEN.

1 H. E. CARLTON ETAL 3,553,003

FIBER REINFORCEMENT Filed Feb. 26, 1968 INVENTORS HERBERT E. CARLTON & ERLAN E. ROSE puf n. 0%

ATTORN EYS United States Patent 3,553,003 FIBER REINFORCEMENT Herbert E. Carlton and Erlan E. Rose, Columbus, Ohio, assignors to The Battelle Development Corporation, Columbus, Ohio, a corporation of Delaware Filed Feb. 26, 1968, Ser. No. 708,166 Int. Cl. C23c 11/00 US. Cl. 117-106 8 Claims ABSTRACT OF THE DISCLOSURE A fiber reinforcement having knob-like end portions and comprised of an inorganic fiber covered with a chemical vapor-plated coating of greater thickness at the ends than therebetween.

This invention relates to a composite fiber reinforcement of unique structure and shape and to its preparation. More particularly, the invention concerns a fiber reinforcement comprised of an inorganic fiber covered with a chemical vapor-plated coating thereover which is of greater thickness at the fibers ends than at the portion therebetween so that in outward appearance an individual fiber reinforcement resembles an elongated dumbbell or a slender rod with rounding knobs on its ends. The invention also concerns preparation of the fiber reinforcement through deposition of a chemical vapor-plating coating on fluidized inorganic fibers.

Numerous plastics, metals ceramics and the like in the past have had fibers, or the like, incorporated therein as reinforcing materials for well-known purposes. As illustrative thereof, there are composites such as metallic fibers in metallic matrices, ceramic fibers in metallic matrices, refractory fibers in ceramic matrices, glass fibers in plastics, and the like. Through use of fiber reinforcements for the matrix material there are obtained generally greatly improved strength-to-density and moduli-to-density ratios in the composite over that of the matrix material and also improvements in other physical and mechanical properties of importance for numerous end applications of the composite. As conventionally employed as reinforcing materials in such composites, the fibers generally have been thread-like or rod-like and of relatively uniform crosssection throughout. Typical useful fibers are of from about A to several inches in length, unless employed as continuous lengths; and have diameters as low as a fraction of a mil, generally less than about 10 mils, and occasionally up to about 100 mils. In some instances, the fiber reinforcements had coatings thereon or comprised a fiber core with a coating thereon. As far as is known prior to the present invention, fabricators in such composite arts have not had access or opportunity to employ fiber reinforcements of the unique structure and shape as taught herein.

:It is to fulfilling this need in the composite art that the present invention is directed. A principal utility of the fiber reinforcement of the invention is as a reinforcing material to prepare composites of fiber reinforcements in various matrices of metals, plastics and ceramics, although other uses for the fiber reinforcement of the invention will be apparent to those skilled in various arts. Because of the unique shape and structure and properties of the fiber reinforcement of the invention, it is contemplated that composites prepared with the same will possess highly unusual and advantageous properties. For example, it is expected that the knobbed ends of the fiber reinforcement will permit a stronger embedment in the composites matrix.

In general, the fiber reinforcement of the invention comprises a core of an inorganic fiber and an outer portion which is a chemically vapor-deposited coating thereon of 3,553,003 Patented Jan. 5, 1971 "ice greater thickness at the ends than over the portion therebetween. The outer portion or chemically vapor-deposited coating constitutes almost always at least 75 percent by volume of the fiber reinforcement. The inorganic core constitutes only a small proportion of the overall fiber reinforcement, almost always in less than about 25 percent by volume of the fiber reinforcement, and most frequently constitutes less than about 10 percent by volume.

The inorganic fiber core usually is threaded-like or rodlike in shape with a length-to-diameter ratio (L/D) of at least 10, more generally and preferably its L/D ratio falls between about 40 and 1200, although inorganic fiber cores of higher L/D ratios are useful for some applications. The inorganic fiber core generally is of circular cross-section, although it may be of other than circular cross-section, rarely exceeds about 3 mils in diameter, and generally and preferably is about 1 mil or less in diameter. The outer portion of the fiber reinforcement, i.e., the chemical vapor-plated coating on the inorganic fiber core, generally over the portion between its ends is of a diameter at least about 1% times the diameter of the fiber core and at its ends is of greater diameter and of a diameter at least about 2 times the diameter of the fiber core. Depending on the particular inorganic fiber and its size, which make up the core of the fiber reinforcement, one generally varies the deposited amount and nature of the chemical vapor-deposited coating to provide a fiber reinforcement of a desired size and properties. For fabricating many composites, an appropriate and desired size for the fiber reinforcement is a length between about A; and 4 inches and a maximum diameter of less than about 50 mils, although fiber reinforcements of from about A to 1 /2 inches in length and of a maximum diameter of less than 10 mils are of greatest utility.

In the drawing, wherein the same numerals identify the same components:

FIG. 1 is a perspective view of the fiber reinforcement of the invention; and

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1.

More particularly, in both of FIGS. 1 and 2, the fiber reinforcement as a whole is designated generally by 10. Fiber reinforcement 10 comprises a chemical vaporplated coating over a core of an inorganic fiber 11, which coating is designated 12 and 12' at the end portions of the fiber reinforcement 10, and is designated 13 for the portion between the ends. The end portions 12 and 12' generally are substantially spherical when prepared by the method herein taught, although they may deviate somewhat from spheroidal shape and still possess advantages of the invention. End portions 12 and 12 also generally blend gradually rather than abruptly into portion 13.

While from the drawing it may appear that the outer portion or chemical vapor-plates coating has a substantially smooth surface, the same is not an essential requirement. The exact surface, smooth, rough or therebetween, can be varied usually as desired through selection of the process for its preparation and/or by variation of process parameters. Generally a smooth outer-surfaced fiber reinforcement is desired as such fiber reinforcements apparently have high tensile strengths. In the particular fiber reinforcement illustrated by FIGS. 1 and 2, the illustrated inorganic fiber is a short length of about 0.5 mil diameter tungsten wire andthe illustrated chemical vapor-plated coating thereover is silicon carbide.

The core of the fiber reinforcement is an inorganic fiber. Numerous inorganic fibers are known and are of utility for the core. The surface of inorganic fiber core should be of a material which is not a reactant in the particularly employed chemical vapor-plating reaction. To mention a few inorganic fibers of utility in the invention, there are boron fibers, silica fibers, carbon fibers,

tungsten fibers, silicon carbide fibers and the like. The

inorganic fiber may be composed of only a single material, such as fine tungsten wire cut in appropriate-size short lengths. It also may be composed of several component materials, such as cut lengths of hot-wire vapordeposited silicon carbide on tungsten wire. Likewise, the useful inorganic fiber may have a thin coating of another material thereon for purposes well known to the art, such as to make it possible to deposit a particular chemical vapor-plated coating on that particular inorganic fiber material. Other inorganic fibers contemplated as useful for preparation of the fiber reinforcement of the invention include: natural mineral fibers, such as asbestos; cut lengths of fine metal wires, such as 3 mil and smaller size Wires of such metals as steel, nickel, copper, etc.; ceramic fibers such as alumina; glass fibers; and the like.

The outer portion, or chemically vapor-deposited coating, of the fiber reinforcement may be of any material which is deposited by a chemical vapor plating process. Thus, the outer portion may be a metal, or alloy, a carbide or carbon, or a nitride, or boron or boride, or silicon or silicide, or oxide, or the like. Pages 57 of Vapor-Plating by C. F. Powell et al., John Wiley & Sons, Inc., New York, 1956, present a tabulation illustrative of numerous coatings which may be chemically vapor deposited and, for purposes of the invention, which are illustrative of the material and materials which can make up the outer portion of the fiber reinforcement. As will be apparent from teachings in that aforementioned book, each and every chemically vapor-deposited coating cannot be deposited on all materials. Thus, depending on the particular material of the inorganic fiber desired as the core of the fiber reinforcement, or alternatively depending on the particular material of the chemically vapor-plated coating desired for the outer portion of the fiber reinforcement, one selects the other so as to permit deposition of a or the particular chemically vapor-plated coating. In some instances, a suitable coating on the inorganic fiber will permit a particularly desired chemical vapor-plated coating to be utilized with an inorganic fiber of a particular material. In other instances, process parameters can be varied or other expediencies known to the art can be employed, to permit deposition of a particularly desired coating.

Particularly useful and preferred fiber reinforcements of the invention comprise those wherein the core may be any of the inorganic fibers of silicon, boron, carbon, tungsten, or silicon carbide, and wherein the outer portion, or chemically vapor-plated coating be a chemical vapor-plated deposit of any of boron, silicon carbide, or carbon.

To prepare the fiber reinforcements of the invention there may be employed a fluidized-bed reactor and a batch-type technique for the chemical vapor plating of inorganic fibers which are fluidized within the reactor during the deposition of coating thereon. Advantages of this process over prior art processes for coating of inorganic fibers include economical production of fiber reinforcements and the particularly incorporated inorganic fiber core in the fiber reinforcement need not be electrically conductive.

Fluidized-bed reactors of utility for carrying forth chemical vapor-plating reactions therein are known to the art with US. Patents Nos. 3,012,876; 3,020,148; 3,043,679; 3,205,042; and 3,234,007, to mention only a few serving to illustrate known teachings for chemical vapor plating of various coatings on various fluidized seed particles and useful fluidized-bed reactors therefor. In general, the prior are fluidized-bed reactors, and those also of utility to prepare the fiber reinforcement of the invention, comprise principally: (a) an upright reactor tube, wherein seed particles (or as in the present invention, fibers) are fluidized and wherein the chemical vapor-plating reaction takes place to coat fluidized seed material within the reactor tube which is of a construction material suitable for the prevailing chemical vapor-plating reaction and particular process parameters employed; (b) means near or at the top of the reactor tube for introducing the particles or other material which are to be fluidized and coated; (c) other means also near or at the top for exhausting the gaseous fluidizing medium, reactant byproducts and the like; ((1) means near or at the bottom of the reactor tube for introducing reactants and generally an inert fluidizing gas; and (e) appropriate heating means and the like, such as electrical heaters or a furnace surrounding the upright reactor and inlet tubes, for heating these tubes and other apparatus components. Such apparatuses also include appropriate means for removal of the product or chemical vapor-coated material and also numerous other auxiliary items, for example, filters, flow meters, temperature controls, valves, piping, etc. with means provided so that the various employed reactants are purified and preheated before introduction into the reactor tube.

In the examples, which follow shortly, there was employed an apparatus of which the fluidized-bed reactor was an about 2-inch diameter upright reactor tube with a a conical bottom having a 5-mm. ID inlet tube fused to the apex of the conical bottom. Surrounding the fluidized bed reactor tube and inlet tube were electrical heating elements and insulation as well as thermocouples so as to determine the temperature of both tubes at various desired locations. There also were provided auxiliary means to control the electrical input to the various heaters and thus to regulate temperature within both tubes. The side wall of conical bottom of the upright tube approximated a cone angle of about 53 with a horizontal plane. At greater cone angles, as of about 90, fibers tended to gather on the cone outside of the gas jet, and at smaller cone angles, as of about 28, the fibers tended to agglomerate in the bottom of the cone. While a 5-mm. ID inlet tube worked Well, with a 3-mm. ID tube, the fibers tended to settle in a ring around the inlet jet, and with a 7-mm. ID inlet tube the fibers tended to agglomerate with the inlet gas flow being sufficient to fluidize them. The upright reactor tube was subjected to vibration of about 120 c.p.s. during fluidization to prevent fibers from sticking to the sidewalls and to aid in their fluidization. Auxiliary apparatus items included sources of a fluidizing gas and those reactants providing the chemical vapor-plated coating along with piping, valves, flowmeters, and the like, for the same so that they could be premixed and introduced into the inlet tube for preheating and flowing upward into the upright reactor tube. The employed ap paratus did not include a distributor plate, screen, or the like for the introduced fluidizing gas and reactants. Instead the inlet gases entered directly as a forceful upwarddirected stream into the conical bottom.

The above-described employed apparatus was designed especially for preparing the fiber reinforcements wherein the inorganic fiber core is tungsten wire of /2 or 1 mil diameter and of a length of about A to /2 inch. In this apparatus fluidized tungsten-fiber densities of from less than 8 up to about 35 fibers/in. were obtainable and able to be maintained. Where the inorganic fiber core is other than of tungsten and of those sizes, as will be apparent to those in the art, appropriate alterations will need to be made in the apparatus to be employed so that the inorganic fibers are suitably fluidized during deposition of the chemical vapor-plating coating thereon. Also as is readily apparent to those in the art, the fluidized-bed process taught herein is readily adapted to scale-up for economical commercial production purposes.

As illustrated by the following wherein silicon carbide was deposited on tungsten fibers, a typical batch-type preparation process can be divided into three stages: the startup, the run, and the shutdown stages. On startup, the reactor was flushed with argon and, with a small amount of argon flowing through the system, the inlet tube preheater and the reactor tube furnace were heated to the predetermined temperatures. In order to precoat the reactor walls with SiC, the hydrogen and CH flows were set to the chosen conditions, and 1 minute later the CH SiCl flow was started at a slow rate and was gradually increased to the rate chosen for the run. After about 20 minutes, the precoating was accomplished and the plating reactants were flushed from the reactor with argon. The argon flow was set to that required for fluidization, usually about 4 liters/min, and the seed fibers were added through the top of the reactor. The hydrogen and methane flows were set to the desired conditions; a minute later the hydrogen flow through the CH SiCl bubbler was started, and was increasedto the desired amount over a period of a few minutes. During the run, the argon flow was adjusted as needed to maintain fluidization, and the other flow rates were maintained constant. Normally, the flow required for fluidization decreased slightly at the beginning of the run because of a decrease in the density of the fiber SiC was deposited on the tungsten fiber seed. Toward the end of the run, the flow requirement increased by about 25 to 50 percent as the fibers became larger. The run was terminated when the fiber diameter reached a desired size such as 4 mils in diameter, as estimated visually during the course of the deposition. At the end of a run, the CH flow and the hydrogen flow to the CH SiCl bubbler were stopped, and a valve between the CH SiCl bubbler and the reactor was closed. A few seconds later, the hydrogen flow was stopped and the fibers were fluidized in argon alone. After about 2 minutes, when it was estimated that substantially all hydrogen had been flushed from the reactor, the argon flow through the reactor was greatly increased, and the fibers were blown out of the reactor and collected in a container.

In the apparatus employed for the examples, the inorganic fibers were fluidized and in a fluidized state when coated, although they tended to follow a particular general path within the upright reactor tube. The fibers appeared to tumble randomly end-over-end and to rise through the central region of the upright reactor tube and then to descend along outer regions of the upright reactor tube with a tendency to have the fibers long axis oriented vertical during descent, before this general path of fluidization repeated itself. It now has not been established whether a particular fiuidization path is essential to prepare the the unique knobbed end fiber reinforcements of the invention by a fluidization technique. Instead it is contemplated that the particular inherent geometry of the inorganic fiber per se at least aids, or most likely is the principle and essential factor, promoting preparation of the unique knobbed-end fiber reinforcements of the invention. Substantially all chemical vapor-plating of coatings on seed particles and the like can be deemed diffusion-controlled processes to at least some extent. As is recognized in such diffusion-controlled processes, the geometry of the article being coated greatly elfects the amount of coating deposited. In the present invention, there is greater access for the diffusing reactants per unit of surface area at the ends of each fiber than per unit surface area of the fiber portion therebetween and this appears to provide deposition of greater amounts of chemical vapor-plating coating at the fibers ends and the resulting knobbed ends. However, whatever an exact explanation be of why the present inventions uniquely shaped fiber reinforcement is produced, it remains that they are made and can be produced by the process herein taught.

The following specific examples are provided to illustrate preferred embodiments, to illustrate the invention in greater detail, and they also serve to provide a better understanding of the invention.

EXAMPLE 1 In this example, the employed upright reactor tube and inlet tube were of quartz. 0.04 g. of tungsten fibers of a length of /2-inch and a diameter of 25 microns were fluidized in the upright reactor tube. The inlet gas composition comprised, in percent by volume, 32% H 62% Ar, 2.3% CH and 3.2% CH SiCl and its total flow approximately 3.8 L/min. when the chemical vapor-plating deposition of the SiC was initiated. The deposition temperature within the reactor tube was controlled to closely approximate 2400 F. and coating of the fluidized fibers was carried forth for about minutes. The coating deposited at a rate of about 0.3 micron thickness per minute. In the central region of the fiber reinforcement the SiC coating approximated 32 microns in thickness, and at the fibers ends the fiber reinforcement approximated an overall diameter of about microns. Hardness measurements on several of the resulting fiber reinforcements averaged 5100 KHN. Tensile strength measurements on several individual fiber reinforcements averaged 235K s.i. by a bend test and 171K s.i. by a direct tensile test. X-ray diffraction analysis of the deposited coated identified the deposited material to be beta-SiC with no other lines observed to indicate the presence of any other crystalline phase in a concentration greater than 5 percent.

The bend test used for tensile measurements in this and other examples which follow served to provide a rapid approximation of the tensile strength of the fiber reinforcement. Briefly the bend test consisted of bending the fiber reinforcements around successively smaller cylindrical mandrels until they broke. The diameter of the smallest mandrel used without breakage is combined with the diameter of the fiber reinforcement and the strength calculated by the following formula (modulus) (diameter of fiber reinforcement) (cylindrical mandrel diameter) wherein the modulus is assumed to be 70,000K s.i. or that of the fully dense material.

The direct tensile test employed a commercial tensiletesting machine. In this test, ends of individual fiber reinforcements were inserted and cemented by an epoxy cement in the bores of two hypodermic needles each connected by a nylon cord loop to a jaw of the test machine. The assembly was aligned before testing and from the determined values, the breaking strength was calculated based on the fiber reinforcements cross-sectional area, assuming it to be a circular cross-section. Diameters of the fiber reinforcement for calculation of cross-sectional area were measured under magnification.

EXAMPLE 2 Additional preparations of fiber reinforcements comprised of 25 microns diameter x /z-inch long tungsten wire fibers coated with chemical vapor-plated silicon carbide were made wherein 0.04 g. of the tungsten wire fibers was fluidized in the upright quartz reactor tube. Typical compositions of the gas introduced through the inlet tube at an initial rate of from 3.6 to 3.8 l./min. were, in percent by volume, 17 to 18% H 80 to 79% Ar,

Strength 0.8% CH and 1.8 to 2% CH SiCl The reactor tube temperature was held at 2300 F. and deposition times of from about 41 to 90 minutes generally were employed. Coating rates of from 0.5 to 0.8 micron/min. thickness were realized. Amounts of coated product of from 0.11 to 0.15 g. were obtained with the amount depending generally on the particular deposition times employed. Measured tensile strength measurements for the fiber reinforcements by the bend test generally averaged from to K s.i., and by the direct tensile test averaged from to 218K s.i. In a few instances tensile strengths as high as 285K s.i. were found. Some fiber reinforcements also had hardnesses as high as 5400 KHN.

From this and similar preparations wherein various process parameters were varied, it was concluded that: the optimum temperature for preparing SiC fiber reinforcements having a tungsten fiber core by thermal decomposition of CH SiCl is between about 22002300 F.; a good quality fiber reinforcement is produced with an inlet gas composition by volume of about 79% H 18% H 1.8% CH SiCl and 0.8% CH the optimum H to CH SiCl ratio is between 10 and 30 and the optimum CH to CH SiCl ratio is between /3 and 1, with, when the first of these ratios is high, the other also should be high; and the deposition rate increases rapidly as the reactor temperature is increased and also increases as the temperature of the introduced gas composition is increased up to that temperature whereat solids would tend deposit therefrom with a useful temperature for the introduced gases being about 1700 to 1750 F.

In these preparations, the resulting fiber reinforcements composed of a tungsten fiber core and the chemically vapor-plated silicon carbide coating thereover possessed the unique shape and structure of the fiber reinforcements of the invention in that the coating was of greater thickness at the fibers ends than at the portion therebetween so that the resulting fiber reinforcement outwardly reassembled on elongated dumbell in appearance.

Metallographic examinations were made of cross-sections of several of the produced fiber reinforcements. In general these metallographic examinations revealed that the nature of the deposited silicon carbide could be varied as desired through selection and variation of process parameters such as deposition temperature, ratio of H to CH and the like. In general the average grain size of the deposited silicon carbide appeared to be less than 0.1 micron and the deposited silicon carbide was obtainable, as desired, in a laminar-structured, or columnar-structured, or mixed laminar-columnar structured deposit, depending on the particular deposition parameters employed. The preferred fiber reinforcements and those of greatest strengths had what appeared to be a combination of a laminar-columnar structural deposit and generally were provided at the determined optimum process parameters.

EXAMPLE 3 In this example, about 0.01 g. of 25 microns diameter by /2-inch long silica fibers were fluidized in the upright reactor tube at a temperature of about 1850 F. while there was introduced therein a gas mixture of hydrogen and methyltrichlorosilane at the rate of about 10-16 l./min. of H and 10 g./hr. of vaporized CH SiCl Duration of the deposition was about 1% hours and there was deposited an approximately 16 microns thick coating composed principally of silicon over the portion between the ends of the silica fibers with a greater deposit of coating at the ends of the produced fiber reinforcements.

EXAMPLE 4 In this example, about 0.01 g. of 40' microns diameter by /2-inch long carbon fibers were fluidized in the upright reactor tube at a temperautre of about 2100 F. while there was introduced therein a gas mixture of hydogen and methyltrichlorosilane at the rate of about 8 to 30 l./min. of H and 1.6 to 37 g./hr. of vaporized CH SiCl over a period of about /2 hour. There was deposited an approximately 40 microns thick coating composed principally of silicon over the portion between the ends of the carbon fibers with a greater deposit of coating at the ends of the produced fiber reinforcements.

EXAMPLE In this example the employed reactor tube is of carbon or carbon-lined, and about 0.01 g. of 25 microns diameter by /2-inch long silica fibers are fluidized therein at a temperature of about 1750 F. while introducing therein a gas mixture of hydrogen and boron trichloride at the rate of 13 l./min. of H and 35 g./hr. of vaporized B01 over a period of about 1 hour. Boron chemically vapor-plated onto the fluidized silica fibers through hydrogen reduction of the boron trichloride so as to deposit a boron coating thickness of about 9 microns or more between the ends of the silica fibers with a greater deposit of the B coating at the ends of the produced fiber reinforcements.

, Since it is obvious that many changes and modifications can be made in the above-described details without de parting from the true nature and spirit of the invention, it is to be understood that the invention is to be limited only as set forth in the appended claims.

What is claimed is:

1. A coated-fiber reinforcement having knob-like end portions and comprised of:

(a) a threador rod-shaped inorganic fiber core;

(b) a chemical vapor-plated coating on said core, which coating is of greater thickness at the ends of the inorganic fiber core than the thickness of the portion therebetween to provide said knob-like end portions, and which coating constitutes at least percent by volume of said reinforcement; and

(c) with said reinforcement between the knob-like end portions having a diameter at least 1% times the diameter of said core, and at the knob-like end portions having a diameter of at least 2 times the diameter of said core.

2. The fiber reinforcement of claim 1 of a length between and 4 inches and of a maximum diameter of less than about 50 mils and whose inorganic fiber core is of a diameter not exceeding 3 mils and has a lengthto-diameter ratio of at least 10.

3. The fiber reinforcement of claim 2 wherein said chemical vapor-plated coating constitutes at least percent by volume of said fiber reinforcement and wherein said inorganic fiber core has a length-to-diameter ratio between 40 and 1200.

4. The fiber reinforcement of claim 3 wherein the coating on said core is a chemical vapor-plated coating of silicon carbide.

5. The fiber reinforcement of claim 3 wherein the coating on said core is a chemical vapor-plated coating of boron.

6. The fiber reinforcement of claim 3 wherein said inorganic fiber core is of tungsten.

7. The fiber reinforcement of claim 3 wherein said inorganic fiber core is of carbon.

8. The fiber reinforcement of claim 3 having a length between A and /2 inch and a maximum diameter of less than 10 mils and whose inorganic fiber core is 1 mil or less in diameter.

' References Cited UNITED STATES PATENTS 3,202,537 8/1965 Norman et al. 117l07.l 3,178,308 4/1965 Oxley et al. 1l7107.2 2,888,375 5/1959 Drummond 1l7l07.1

ALFRED L. LEAVI'IT, Primary Examiner W. E. BALL, Assistant Examiner US. Cl. X.R. 1l7-l00, 107

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,003 Dated January 5 1971 Herbert E. Carlton et 211. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 16, "235" should read 12S Signed and sealed this 20th day of April 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

